Are there any advantanges to VB theory over MO theory?

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In summary, the conversation discusses the comparison between valence bond theory and molecular orbital theory in explaining various chemical phenomena. It is mentioned that while VB theory can explain some things that MO theory cannot, such as the stability of F2 and back bonding in metal complexes, it becomes cumbersome when dealing with metallic or semi-metallic compounds. On the other hand, MO theory is more accurate but also more computationally expensive. The conversation also mentions the application of VB theory in explaining the Woodward-Hoffmann rules and the concept of resonance structures in coordination complexes.
  • #1
CrimpJiggler
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Are there any things that can be explained by valence bond theory, that cannot be explained by molecular orbital theory? I know of various things that can only be explained by MO theory, and not VB theory, but does it go both ways?
 
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  • #2
Yes, e.g. VB yields a molecule F2 while it isn't stable in Hartree Fock theory.
I don't think there are many things that can't be explained in VB theory but only in MO theory.
However, VB theory becomes cumbersome when you use it in metallic or semi-metallic componds
like Boranes as you need an excessive number of resonance structures.
Generally, VB calculations are much more accurate than Hartree Fock calculations but also much more
costly in terms of computing power.
 
  • #3
One case that I know of that can be explained only by MO theory, are back bonding in carbonyl metal complexes (the C=O bond gets weakened by electron donation from the metal, AFAIK this can only be explained by considering antibonding orbitals). Another example is cycloadditions, i.e. you can explain why photoexcited 2 + 2 cycloadditions work, by considering the phases of the MOs. Maybe these things can be explained by VB theory too, I haven't put too much thought into it.
 
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Thanks. Yeah, resonance structures explain it pretty simply alright. I didn't know resonance structures applies to coordination complexes like that. That'll help me a lot in understanding the properties of various complexes, thanks a lot.
 

What is the difference between VB theory and MO theory?

VB (Valence Bond) theory and MO (Molecular Orbital) theory are two different approaches to understanding the bonding between atoms in a molecule. VB theory describes bonding as the overlap of atomic orbitals, while MO theory describes bonding as the combination of atomic orbitals to form molecular orbitals.

Which theory is more accurate?

It is difficult to say which theory is more accurate, as both have their strengths and limitations. VB theory is better at explaining the shapes of molecules and predicting bond angles, while MO theory is better at predicting the energies of bonding and antibonding orbitals.

Are there any advantages to using VB theory over MO theory?

One advantage of VB theory is that it is easier to visualize and conceptualize, making it more intuitive for beginners to understand. It also allows for the consideration of resonance structures, which MO theory does not. Additionally, VB theory is better suited for describing localized bonding, such as in covalent compounds.

What are the limitations of VB theory?

VB theory is limited in its ability to predict the energies of molecular orbitals, making it less accurate for more complex molecules. It also does not account for delocalized bonding, which is important in molecules with conjugated systems.

When should VB theory be used instead of MO theory?

VB theory is often used for simple molecules with localized bonding, such as diatomic molecules and covalent compounds. It is also useful for understanding the concept of resonance in molecules. However, for more complex molecules with delocalized bonding, MO theory is a better approach.

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